Navigation

Category Archives: Pyroclastic flow

Post navigation

The people who remain in Chaiten face the potential for a devasting pyroclastic flow, so says Jorge Muñoz of the SERNAGEOMIN in Chile. The volcano is still producing large ash columns on Tuesday and a flyover of the dome forming inside the caldera has lead to the concern that a collapse on a larger scale than those seen last week could wipe out the town for good.
The government hopes news like this from volcanologists might convince the last remaining residents of Chaiten to leave, but no indication of this has come to pass. In fact, things sound like they’re getting heated in the fight over the town of Chaiten. The Minister of Government Affairs had this to say to the lawyers for the remaining residents:

“I think that up until now, we have been quite convincing, but to say that the problem is to unblock the river (…) Why don’t we back up a bit? Why is the river overflowing? Because the volcano is exploding, that’s why!”

It gets even uglier here from the Undersecretary of the Interior. It seems that both sides are digging in the heels, so to speak.

It is hard to believe that the eruption at seem to come out of nowhere at Chaiten started over 8 months ago now, and apparently is still not showing many signs of abating. I did get a chance to see some great talks and posters at AGU last month about the Chaiten eruption, with the key points I took away being that Chaiten is erupting a very crystal poor rhyolite (<1% crystals) and that it seems that the source of the magma is relatively deep in the Andean crust. Also, there are some indications that the eruption at Chaiten may have been tectonically instigated – i.e., that earthquakes in the area might have helped the magma to erupt – at least that is what Luis Lara of the SERNAGEOMIN believes (hat tip to Thomas Donlon for the link). The eruption at Chaiten also wreaked more havoc on aviation in South America than we thought, effecting airports 1000s of kilometers away and almost bringing down a number of aircraft. Most everyone I talked to seems to think what we are seeing is very similar to what happened at Little Glass Mountain in California about 1,000 years ago.
Moreover, the eruption hasn’t really stopped since it began in May of 2008. In fact, just last week we saw a collapse of part of the new dome that have produced some pyroclastic flows within and outside the caldera (see above and the Volcanism Blog) and fed more ash into the choked rivers near the volcano. It is anyone’s guess (well, at least at AGU) how long this eruption might go on – weeks? months? years? – but the consensus is that this might be a once-in-a-lifetime eruption (but we already knew that, didn’t we?)

I just got back from a 4 day field trip to the Long Valley Caldera in eastern California, so I’m a little behind on posting. The field trip was great and I had a chance to see a lot of pumice, a lot of welded tuff (in the form of the Bishop Tuff, the large ignimbrite that erupted from the Long Valley Caldera ~760,000 years ago) and got to lead the part that looked at the Mono domes (too bad snow covered the Inyo Domes). Pictured above some puffed obsidian – the layers are obsidian and vesicle-rich obsidian – that were erupted, cooled, expanded and cracked (known as “breadcrusting“). This is part of Panum Crater, the youngest of the Mono domes that comes in at ~600-650 years old.

After nearly a year and a half of little lava dome growth at Soufriere Hills in the West Indies, this past July 26th, the volcano erupted new dome material. Why do I remind us of this, you ask? Well, the Scientific Advisory Committee at Montserrat feel that the dome growth might be restarting in earnest:

Since August, any new supply of lava has been minimal. Thus while there is evidence that the eighteen-month long pause in lava dome growth may be coming to an end, it has not happened yet.

The most troubling event, in their minds, is the new activity at the Gages Wall vent, which means that new dome growth might be possible further west into the Gages Valley (marked green on the map below. The upshot? Plymouth will be put into much greater danger of pyroclastic flows produced by dome collapses. Most of the pyroclastic flows travel to the north and south of Soufriere Hills’ summit, so a change in the dome to focus down the Gages Valley would change the hazard prediction game quite a bit at Soufriere Hills.

Sorry about the dearth of posts. It has been a busy week here in Davis and I’ve been a little distracted by the upcoming election. Combined with the relative lack of volcano news this week, the posting has been lackluster.
However, that being said, I will try to make up for some of it by starting my Volcano Profiles series that will bide the time between volcano news. I start with a volcano that was suggested by Eruptions reader Thomas Donlon: Rabaul.

VOLCANO PROFILE: RABAUL

Location: Papau New Guinea

Height: 688 m

Geophysical location: Boundary of Australian plate and Pacific plate, where the Pacific plate is subducting under the Australia (note: very simplified as there are a number of microplates involved as well).

Summary: Rabaul is an 8×14 km caldera that has been partially filled by the sea, but multiple peripheral vents can been seen along the caldera edges. Likely the caldera seen today was formed 1,400 years ago and there is evidence that an ancestral caldera formed 7,100 years ago. The volcanic system erupts basaltic through dacitic lavas from these vents, producing lava flows and voluminous pyroclastic flows that have caused extensive death and damage to the settlements near the caldera over the last 200 years. The currently active vents are Tavurvur and Vulcan.

Current status: The Rabaul caldera is currently in an active cycle which started in 1994, producing ash and steam eruptions, along with lava flows and ash flows from both Tavurvur and Vulcan. The caldera has been constantly active since 2006, with the most recent activity consisting of earthquake and small, conduit-clearing eruptions from a shallow reservoir.

Notable Recent Eruptions: The Rabaul Caldera has had two major eruptions in the past 100 years and they are stark contrasts in terms of hazard mitigation and evacuation. The eruptions from Vulcan and Tavurvur in 1937 killed more than 500 people while blanketing the countryside in ash. It also produced a tsunami that washed boats onshore. This eruption is thought to have been roughly a VEI 4. Vulcan and Tavurvur followed up this eruption with another VEI ~4 in 1994, but thanks to excellent planning, evacuating practice and an organized response, the death toll to this eruption was a mere 5 (an excellent summary of this evacuation was presented on an episode of NOVA). This eruption produced ash fall and pyroclastic flows from at least 5 vents along the caldera and the ash fall effects were amplified by heavy rain (likely caused by the eruption itself). Some of this ash was in excess of 7 feet and destroyed 80% of the structures in the town of Rabaul. Interestingly, the only real signs of a potential eruption at the Rabaul Caldera before 1994 was significant surface uplift in the caldera in the mid-1980s, followed by quiescence until the 1994 eruption.

UPDATE 10.9.08: This photo is not from the current eruption (see the comments below), so disregard any comments I might have about the current state of activity.
Contrary to some of the earlier reports, the eruption at Soputan in Indonesia might be more impressive than previously though. Pictures of the eruption (above) show a fairly healthy eruption column coming from the volcano, although it is hard to tell if the image is just an eruption column or a column with an associated column-collapse pyroclastic flow heading down the far side of the volcano (in the picture). The former would imply a vigorous supply of gas-rich magma, the latter might imply less magma in the conduit with an eruption rate less than is needed to support the ash column. In any case, Indonesia officials are evacuating a 4-km radius from the volcano as it continues to erupt.

Two volcanoes on the island of Luzon look like they might be primed for activity, at least according to PHILVOLCS, the Philippine Institute of Volcanology and Seismology. Both Mayon and Taal (above) are showing signs of increased seismicity. Only Mayon is actively steaming right now and the volcano had a mild ash eruption on August 10 as well. Officials in the Philippines have both volcanoes on Alert Level 1 (potential activity).
Taal is actually part of a bigger system that occupies a 15 by 20 km caldera. The island where most historic activity has occurred is made up of a series of smaller stratocones and the volcano itself is one of the more dangerous in the Pacific Rim. The last eruption of Taal was over 30 years ago, but it has a long history of eruptions that produce pyroclastic flows and lava flows.

It seemed like events at Soufriere Hills had been heading for a dome collapse for the past few week and sure enough, part of the summit dome collapsed last night, producing pyroclastic flows that reached the ocean along with a 40,000 foot ash column. The report I received from the Montserrat Volcano Observatory offered the following details:

The collapse started at 11:27 pm local time on Monday 28th July 2008
without any precursory activity. Part of the western side of the lava
dome collapsed generating pyroclastic flows that reached Plymouth and
the sea.
There were also a few explosions from the dome during the collapse, with
the largest at approximately 11:32 pm.
The height of the ash column was estimated at 12 kilometres (40,000
feet) above sea level.

Luckily, unlike the eruption in 1997 that killed 17 people, there were no reported fatalities or injuries related to this dome collapse event. The dome collapse itself is merely the failure of the oversteepened lava dome at the summit of Soufriere Hills but the avalanche becomes a pyroclastic flows because the material in the dome is still hot and mixes with whatever volcanic gases and ash are being expelled from the vent as well. This is the modus operandi of Soufriere Hills – at least in the last 10 years of eruptions at the Caribbean volcano.